The present invention relates to a method of and apparatus for wire electric-discharge machining.
FIG. 14 shows the mechanism of general electric-discharge machining which is popularly used in the field of mold machining in the automotive industry, the home electric industry, the semiconductor industry, and the like. When a pulse-like voltage is applied to an inter-pole gap between an electrode and a workpiece which are dipped in a machining liquid, the processes are sequentially performed, such as (1) formation of an arc column by generation of discharge and local melting by discharge heat energy, (2) to (3) generation of evaporative explosive power of the machining liquid and scattering of the melted portion, and (4) to (5) cooling and solidification of the melted portion by the machining liquid and recovery of inter-pole insulation. These processes are repeated at a high frequency, and thereby the workpiece can be machined. In electric-discharge machining, an inter-pole gap between the electrode and the workpiece is kept to be very small, i.e., several xcexcm to several ten xcexcm. This is an important factor of precise machining.
Of such discharge machining techniques, a target of this invention is a wire discharging machining technique which machines a workpiece by using a wire electrode. The wire electric-discharge machining technique is used in piercing machining, cutting machining, and the like, and a demand for precise machining is especially enhanced. For example, a precise mold used in the semiconductor industry, a high precision of 1 to 2 xcexcm has been demanded.
FIGS. 15A, 15B, and 15C show machining processes performed in the wire electric-discharge machining. As shown in FIG. 15A, in the wire electric-discharge machining, rough machining called first cutting is performed first. This first cutting is machining in which a wire electrode is passed through an initial hole, and a workpiece is cut by the wire electrode. In general, in the first cutting, in order to perform finishing after the first cutting, severe surface roughness and severe precision are not demanded, but the most important factor is that a machining speed is increased. In order to increase the machining speed in the wire electric-discharge machining, a machining liquid is strongly sprayed so that machining waste is efficiently exhausted from the inter-pole gap between the wire electrode and the workpiece. In order to uniformly spray the machining liquid and to prevent disconnection of the wire electrode, a method of dipping the workpiece in the machining liquid collected in a machining tank is used.
Upon completion of the first cutting, a core (scrap) is removed, and, as shown in FIG. 15B, middle finishing called second cutting is performed. In addition, as shown in FIG. 15C, finishing called third cutting is performed. By the way, the second cutting and the third cutting are called for convenience in easy understanding, which means that all the machining procedures are not always completed in three processes. Some are completed by performing the second cutting, but some requires third or more processes when demands for surface roughness and dimensional precision are severe.
In finishing after the second cutting, surface roughness is made fine to adjust the shape. Therefore, when an amount of residue on a target shape is uneven, the shape is corrected, but when an amount of residue on a target shape becomes even, machining which uniformly removes the workpiece must be performed. When such finishing is performed, so-called xe2x80x9celectrode position servoxe2x80x9d, which controls drive speeds serving as a speed of relative movement of the wire electrode and the workpiece, is performed such that the drive speeds are equal to a predetermined value based on an inter-pole voltage between the wire electrode and the object to be machined.
On the other hand, in the wire electric-discharge machining, an adaptive control to prevent disconnection of the wire electrode which causes machining interruption is performed. As an adaptive control to prevent disconnection, the most ordinary method is one that changes a discharge stop time to decrease machining energy.
As explained above, in the wire electric-discharge machining, a plurality of control methods are often applied to a mechanical system, a power supply system, and the like. However, the application of the plurality of control methods may unexpectedly produce an inconvenience in a machining result.
An example of such an inconvenience will be explained below with reference to a wire electric-discharge machining apparatus shown in FIG. 16. In FIG. 16, reference numeral 1001 denotes a wire electrode, 1002 a workpiece, 1003 a movable table, 1004 a machining power supply, 1005 a machining power supply control unit, 1006 a control unit, 1007 a servo mechanism, and 1008 denotes an inter-pole voltage detection unit.
The control unit 1006 sends a machining condition signal to the machining power supply control unit 1005 according to an input machining condition. The machining power supply control unit 1005 drives a switching element (not shown) of the machining power supply 1004 according to a signal from the control unit 1006. The machining power supply 1004 applies a pulse-like voltage across the wire electrode 1001 and the workpiece 1002 to perform electric-discharge machining to the workpiece 1002. An inter-pole voltage between the wire electrode 1001 and the workpiece 1002 in the electric-discharge machining is detected by the inter-pole voltage detection unit 1008 to be sent to the control unit 1006. The control unit 1006 determines a drive speed of the movable table 1003 based on inter-pole voltage information sent from the inter-pole voltage detection unit 1008 and sends a command to the servo mechanism 1007. As a result, the servo mechanism 1007 allows the movable table 1003 to move at the drive speed, and the wire electrode 1001 and the workpiece 1002 relatively move.
In general, a positional control of the wire electrode 1001 is performed based on a measured voltage in the inter-pole voltage detection unit 1008. As a control method used in this case, the following method is used. That is, drive speeds serving as a speed of relative movement between the wire electrode 1001 and the workpiece 1002 are increased when the inter-pole voltage is high, and the drive speeds are decreased when the inter-pole voltage is low. However, it has been understood that the control method for the drive speeds cannot be used without various problems.
In the first cutting, it is mainly required that a machining speed is increased as explained above. Disconnection which is a factor of hindering the increase in machining speed must be avoided as much as possible. Research and development are earnestly performed to prevent disconnection. Although various methods are reported and practically used, a method which is the most effective method is that a discharge stop time is elongated.
However, since a power supply control which elongates the discharge stop time responses considerably quickly as compared to a drive control for the mechanical system, a vibration phenomenon may occur due to the difference between respective responses of both the controls.
The wire electric-discharge machining apparatus which performs a control to prevent the wire electrode 1001 from being disconnected recognizes a decrease in inter-pole voltage as a leading phenomenon in which the wire electrode 1001 is disconnected. Therefore, when the inter-pole voltage lowers, the operation of elongating a discharge stop time is performed. This operation is effective as means for preventing disconnection. When the discharge stop time becomes long, the inter-pole voltage more lowers and thereby the apparatus erroneously recognizes that the wire electrode 1001 is about to be disconnected, and performs the operation of further elongating the discharge stop time. Such a series of operations produces a vicious circle. In contrast to this, when the inter-pole gap increases to raise the inter-pole voltage, the machining power supply control unit 1005 performs a control which shortens the discharge stop time. When the discharge stop time becomes short, the inter-pole voltage becomes high. Therefore, the apparatus erroneously recognizes that the inter-pole gap becomes further large and performs an operation of further shortening the discharge stop time.
On the other hand, in finishing performed after the second cutting, machining energy is not so increased, and therefore the phenomenon in which the wire electrode 1001 is disconnected does not occur unless any trouble occurs. Accordingly, such a problem may not occur frequently. In the finishing performed after the second cutting, since it is required that the workpiece 1002 is uniformly removed, an inconvenience may occur with the requirement.
For example, it may be desired that a machining removal amount of the workpiece is decreased through wire electric-discharge machining to keep the precision of shapes such as an outside corner or the like of a sharp edge. At the outside corner, the wire electrode 1001 is stayed for a long period of time in a range which is very close to the outside corner. For this reason, the machining removal amount of the workpiece is often excessive with respect to a desired shape. In such a case, it is general that the discharge stop time is elongated to decrease the machining removal amount.
However, when the discharge stop time is elongated in order to decrease the machining removal amount, the inter-pole voltage decreases, and the drive speed becomes low. Accordingly, the machining removal amount does not decrease as it is desired. Similar examples include a contact between a portion of a shape to be machined and a portion called an approach which leads to the shape.
As explained above, when the control for the mechanical system and the control for the power supply system are simultaneously performed, the controls become easily unstable, and a machining removal amount may not precisely be controlled.
It is an object of this invention to obtain a method of and apparatus for wire electric-discharge machining which can precisely controlling a machining removal amount in finishing machining and improving machining precision.
The wire electric-discharge machining apparatus according to one aspect of this invention comprises a control unit which determines a speed of relative movement between a wire electrode and a workpiece based on a voltage corresponding to an inter-pole voltage between the wire electrode and the workpiece. The wire electric-discharge machining apparatus further comprises a machining unit which generates a discharge between the wire electrode and the workpiece and relatively moves the wire electrode and the workpiece at the speed of relative movement determined by the control unit to process the workpiece. The wire electric-discharge machining apparatus also comprises an inter-pole voltage correction unit which corrects the voltage corresponding to the inter-pole voltage according to a correction coefficient which increases depending on an increase in discharge stop time. The corrected voltage corresponding to the inter-pole voltage output from the inter-pole voltage correction unit is given to the control unit to relatively move the wire electrode and the workpiece according to the corrected voltage corresponding to the inter-pole voltage.
According to the above-mentioned aspect, when a discharge stop time becomes long, correction is performed by the inter-pole voltage correction unit such that a voltage corresponding to an inter-pole voltage increases, and the wire electrode and the workpiece relatively move depending on the corrected voltage corresponding to the inter-pole voltage.
The wire electric-discharge machining method according to another aspect of this invention comprises the steps of generating discharge between a wire electrode and a workpiece, determining a speed of relative movement based on a voltage corresponding to an inter-pole voltage between the wire electrode and the workpiece, and relatively moving the wire electrode and the workpiece at the speed of relative movement to process the workpiece. The wire electric-discharge machining method further comprises the steps of elongating a discharge stop time when a machining removal amount is partially reduced, and of correcting the voltage corresponding to the inter-pole voltage depending on the elongated discharge stop time.
According to the above-mentioned aspect, the machining removal amount per unit time can be decreased by elongating the discharge stop time without decreasing the speed of relative movement between the wire electrode and the object to be machined.
The wire electric-discharge machining method according to the still another aspect of this invention comprises the steps of generating discharge between a wire electrode and a workpiece, determining a speed of relative movement based on a voltage corresponding to an inter-pole voltage between the wire electrode and the workpiece, and relatively moving the wire electrode and the workpiece at the speed of relative movement to process the workpiece. The wire electric-discharge machining further comprises the steps of elongating a discharge stop time at a crossing portion between a contour forming path of the wire electrode in the workpiece and an approach path reaching the contour forming path in comparison with another portion, and correcting the voltage corresponding to the inter-pole voltage depending on the elongated discharge stop time.
According to the above-mentioned aspect, the machining removal amount per unit time can be decreased by elongating the discharge stop time without decreasing the speed of relative movement between the wire electrode and the workpiece, at the crossing portion between the contour forming path of the wire electrode in the workpiece and the approach path reaching the contour forming path.
The wire electric-discharge machining method according to still another aspect of this invention comprises the steps of generating discharge between a wire electrode and a workpiece, determining a speed of relative movement based on a voltage corresponding to an inter-pole voltage between the wire electrode and the workpiece, and relatively moving the wire electrode and the workpiece at the speed of relative movement to process the workpiece. The wire electric-discharge machining further comprises the steps of elongating a discharge stop time at a corner portion of the workpiece in comparison with another portion, and correcting the voltage corresponding to the inter-pole voltage depending on the elongated discharge stop time.
According to the above-mentioned aspect, the machining removal amount per unit time can be decreased by elongating the discharge stop time without decreasing the speed of relative movement between the wire electrode and the workpiece, at the corner portion of the workpiece.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.